The present invention relates generally to fluid pipeline passages and more particularly to an improved fluid loading arm which suppresses harmful cavitation without significant increase in system back pressure.
At the present time, it is well known that flow velocities in loading arms, and particularly in marine fluid loading arms, are restricted by the phenomenon of "cavitation." This occurs in the loading arm short radius elbows. Thus, as a fluid flows through an elbow there are pressure gradients which occur across the elbow cross-section and the pressure is lowest at the inner or shortest radius. As the velocity of the flow increases, the pressure at the inner radius will decrease until the vapor pressure of the fluid has substantially been reached. At this particular point in time a plurality of relatively small bubbles or cavities of the product vapor will be formed and begin to flow downstream and when the bubbles reach a region of relatively higher pressure, the bubbles will implode, in other words collapse, with a violent release of energy. These implosions can cause erosion of the pipewall and vibration in the arm, which can then result in arm failure because of structural weakening, fatigue failure from the dynamic stresses produced, or as a minimum, costly replacement of eroded arm components.
Generally, it is economically desirable and there exists significant incentive to increase the flow velocity through the arm. The advantages of increased flow are a shorter turnaround time at an existing or new berth, and avoiding the need to replace existing arms with larger diameter arms, as well as in the case of new terminals smaller and/or fewer arms can be used. This results in a reduction of capital investment. However, when these flow velocities are increased this can cause a corresponding increase in damage to the arm because of the low pressure zones created at the inner radius of the elbows. Thus, while increased flow velocity is desirable, the corresponding problem of cavitation created thereby is a primary concern.
The primary parameters which determine when cavitation will occur include the flow variables of absolute pressure, P.sub.o, and velocity, V.sub.o, the boundary geometry and the vapor pressure, P.sub.v, and density, .rho., of the liquid. The relationship between these parameters is generally known as the cavitation number, sigma, which equals EQU (P.sub.o -P.sub.v)/(1/2.rho.V.sub.o.sup.2)
Thus, if one could increase the flow velocity, the foregoing and other advantages will be obtained by not without the attendant disadvantages, unless means are introduced to suppress cavitation.
The systems and arrangements currently used in the elbows for suppression of cavitation and improving flow rate comprise turning vanes in the arm elbows. These vanes split the single flow stream into a multiplicity of streams, each of which has smaller pressure gradients than the overall pressure gradient of the single flowing stream. This permits a higher pressure rating to be achieved with a vaned elbow before the vapor pressure of the fluid is reached and without producing cavitation. This arrangement is generally known in the art as typified by the disclosures of U.S. Pat. Nos. 1,837,901; 1,996,596; 2,662,553; 2,723,680; 2,733,044; 3,597,166. A patent of particular interest is U.S. Pat. No. 3,724,499 wherein there is disclosed a loading arm having shaped turning vanes in the elbows in combination with a Venturi section at the outlet of the loading arm for raising the fluid pressure in the arm. The concept of increasing arm fluid pressure is well-known in the industry. As previously discussed, it will permit higher flow velocity to be achieved before cavitation occurs. Thus, in the prior art patented loading arm design it is necessary to have increased back pressure to cause the improved flow rate. The same effect can be obtained as well by employing a valve at the outlet pipe for restricting flow. However, a disadvantage of that type of system would be the additional pump capacity which is required in order to overcome the additional back pressure due either to the Venturi or the valve. Other prior art patents of general interest are U.S. Pat. No. 2,564,683 which discloses adjustable plates in a flow passage and U.S. Pat. No. 3,398,765 discloses an elbow-located guide plate for minimizing the effect of the bend on flow. Thus, it is apparent that there is a need for a system which will permit higher flow rate through the fluid loading arm without cavitation and without a significant increase in the back pressure.